Indium phosphide (InP) crystals are manufactured by liquid encapsulated Czochralski method (LEC method) or vapor pressure controlled LEC method (VCZ method). Recently, growth of monocrystals of 3 inches diameter (approximately 75 mm) and 4 inches diameter (approximately 100 mm) by vertical gradient freezing method (VGF method) has been reported.
With the VGF method, it has been reported that because crystals are grown under a low temperature gradient, InP crystals with low dislocation density can be grown. For example, in 10th International Conference on Indium Phosphide and Related Materials, Post Deadline Papers, Tsukuba, Ibaraki (1998) 15-16, there is reporting of an Fe-doped InP crystal of 3 inches diameter. In this paper, it is reported that the etch pit density (EPD) of (100) wafer was 3,000 cm−2. This etch pit density corresponds to the dislocation density of the crystal. In this paper, the growth orientation of the crystal is not shown. In Technical Digest of GaAs IC Symposium, Monterey, (2002) 147-150, with a commercial Fe-doped (100) InP wafer of 4 inches diameter, there was a large gradient in the etch pit density and photoluminescence (PL) intensity on the wafer, and Fe concentrations changed approximately two-fold. From this, the growth orientation of the commercial VGF crystal was presumed to be <111>. In addition, when Fe-doped InP crystals of 4 inches diameter are grown by the vertical boat method using a <100> seed, it has been reported that a (100) wafer with a dislocation density average value of 11,000 cm−2 was obtained.
In addition, in the 10th International Conference on Indium Phosphide and related Materials, Post Deadline Papers, Tsukuba, Ibaraki (1998) 1-2, Japanese Journal of Applied Physics, 38 (1999) 977-980, there is reporting of an InP crystal of 100 mm diameter which was grown in the <100> orientation by VGF method. Furthermore, in the 11th International Conference on Indium Phosphide and Related Materials, Davos, Switzerland, (1999) 249-254, InP crystals of 100 mm diameter grown in the <100> orientation by VGF method were heat treated in an iron phosphide atmosphere to obtain Fe-doped (100) InP wafers of 100 mm diameter.
In addition, in Journal of Crystal Growth 132 (1993) 348-350 and Journal of Crystal Growth 158 (1996) 43-48, using a <100> oriented seed crystal which has approximately the same diameter as the crystal body and adding sulfur (S), a monocrystal of 50 mm diameter was obtained.
In the growth of InP crystals, twin generation is the most serious problem. In particular, with the vertical boat method in which crystals are grown in containers such as VGF method and vertical Bridgman method (VB method), when crystals are grown under a low temperature gradient, there is a high frequency of twin generation, and it is extremely difficult to obtain a monocrystal.
As a result, in Journal of Crystal Growth 94 (1989) 109-114, there is reported a method of growth in the <111> orientation in which twins are not readily generated. However, as described in Technical Digest of GaAs IC Symposium, Monterey (2002) 147-150, in order to use the usual (100) wafers, the (100) wafer must be sliced at an angle of 54.7 degrees with respect to the growth direction. As a result, there results a large gradient for the dopant concentration on the wafer. Commercial Fe-doped (100) InP wafers of 4 inches diameter (approximately 100 mm) have been reported to have approximately two-fold changes in Fe concentration on a wafer. When there is such a large change in Fe concentration, there are also large changes in electrical properties on the wafer. As a result, when this is used for optoelectronic devices such as semiconductor lasers for optical communication, photodetectors, and the like, and for electronic devices such as transistors and the like, the performance of the device is not constant on the wafer.
On the other hand, as described in Japanese Laid-Open Patent Number 11-302094, in order to prevent the generation of twins, the crystal growth rate at a tapered part is preferably 20 mm/hr or greater, and the slope angle for the tapered part of the inverse-conical crucible is 80 degrees or greater and less than 90 degrees with respect to the crystal central axis. Normally, the dopant is placed together with the raw material in the crucible, and crystal growth is conducted. However, if the growth speed is too fast, constitutional supercooling occurs, which results in polycrystallization. As described in 11th International Conference on Indium Phosphide and Related Materials, Davos, Switzerland, (1999) 249-254, a monocrystal in which dopant is not added is grown, and after making this into a wafer, heat treatment is conducted under an iron phosphide atmosphere in order to obtain an Fe-doped InP substrate. However, with this method in which dopant is diffused from the atmosphere, this may result in dopant concentrations higher in areas closer to the wafer surface. As a result, when using for optoelectronic devices such as semiconductor lasers for optical communication, photodetectors, and the like, and for electronic devices such as transistors and the like, the device performance may not be stable.
In addition, in Journal of Crystal Growth 158 (1996) 43-48, by using a <100> oriented crystal seed with a diameter approximately equal to that of the crystal, it is reported that non-doped or sulfur (S) doped monocrystal of 50 mm diameter was obtained. However, despite adding S, which has the effect of reducing dislocation density, at a high concentration of 2×1018cm−3, the etch pit density (EPD) was high at 8,000-10,000 cm−2. With an InP substrate used in the optoelectronics field such as semiconductor lasers for optical communication, photodetectors, and the like, dislocation reduces the device performance and life span. Substrates with such a high dislocation density are problematic for practical use.